Anti-lock braking systems, commonly referred to as ABS systems, are found increasingly in motor vehicles of all types including trucks. ABS systems function by monitoring the rotational speed of a wheel and using that information to adjust the braking force applied to the wheel to avoid lock-up of the wheel during braking. ABS systems typically employ a sensor mounted on a non-rotating portion of the axle, adjacent a rotating portion of the wheel assembly, to determine the wheel rotational speed.
In many ABS systems, the sensor used is a magnetic pick-up. The sensor produces a voltage signal when excited by a ferrous tone ring or exciter ring which is mounted on a rotating portion of the wheel assembly. The tone ring includes a series of teeth which project from a surface of the tone ring. The sensor is mounted adjacent the tone ring, and each time a tooth of the tone ring rotates past the magnetic pick-up an electrical pulse is generated. The electrical pulse is sent to a logic portion or computer associated with the ABS system. The computer associated with the ABS system evaluates the pulses to determine, among other things, the speed and acceleration of the rotating wheel and whether wheel lock-up is impending. In response to this determination, the ABS system modulates the braking forces to prevent wheel lock-up from occurring.
It is desirable to be able to determine when a vehicle is backing up, so as to activate a back-up alarm or other feature of the vehicle. This can be especially desirable in a tractor-trailer combination because the overall length of the combination places the operator of the vehicle from being close to and seeing around the rear portion of the vehicle. In a typical commercial tractor-trailer combination, there is no wiring dedicated between the tractor and the trailer for carrying a signal for use in activating a back-up alarm of the vehicle. Adding a dedicated wire for this purpose would be difficult commercially. Therefore, it would be desirable for any such system not to require dedicated wiring between the tractor and the trailer.